1. Field of the Invention
The present invention relates to image forming apparatus, and more particularly to an image forming apparatus using a semiconductor laser.
2. Description of the Related Art
Conventionally, digital copying machines, laser beam printers and like image forming apparatus have a semiconductor laser as a light source. FIG. 1 shows the relationship between the drive current I of the semiconductor laser for use in such apparatus and the radiation output P thereof. As the ambient temperature T varies to T.sub.1, to T.sub.2 and to T.sub.3 (T.sub.1 &lt;T.sub.2 &lt;T.sub.3), the threshold current I.sub.th at which the laser starts oscillation increases with the rise of the ambient temperature T. The temperature dependence of the threshold current I.sub.th is represented generally by the following expression (a) wherein T.sub.0 is the characteristic temperature intrinsic to the semiconductor laser. EQU I.sub.th 'exp T/T.sub.0 (a)
Accordingly, the lower the characteristic temperature T.sub.0 of the laser, the greater is the temperature dependance. Further when the drive current to be supplied to the laser has a constant value I.sub.0, changes in the ambient temperature T result in variations in the radiation output P (variation range: .DELTA.P).
Stripe semiconductor lasers which are used most prevalently are low in characteristic temperature T.sub.0 and therefore have great temperature dependence. For this reason, it is likely that the radiation output variation range .DELTA.P will exceed an allowable range of the image forming process. To preclude this, an auto power control (APC) circuit is provided for controlling the drive current I of the semiconductor laser based on the radiation output P detected.
For the detection of the radiation output, however, the control by the APC circuit requires the emission of light by the laser irrelevant to image formation to shorten the life of the laser. Moreover, the provision of the APC circuit itself leads to an increased cost.
Besides stripe semiconductor lasers, various types of semiconductor lasers are known which include, for example, quantum-well semiconductor lasers having a high characteristic temperature T.sub.0 and relatively low in temperature dependence. The quantum-well semiconductor laser nevertheless has a great differential efficiency .eta. (inclination of straight line in the laser oscillation region) and therefore has the drawback that the radiation output variation range .DELTA.P exceeds the allowable range of the image forming process despite the small temperature dependence.